FIG. 1 illustrates a portion of a UMTS wireless communication network. As shown, user equipment UE wirelessly communicates with a Node-B serving the communication needs of a geographic area (often referred to as a cell or collection of cells). A UE may be a mobile phone, wireless equipped PDA, wireless equipped laptop, etc. UEs are also commonly called mobile stations or mobile units. Node Bs are network stations and are also commonly called base stations. Node-Bs communicate with a radio network controller (RNC), which may configure the UE and the Node B for communication over various channels.
As part of the release 6 versions of the 3GPP specifications, the feature of an F-DPCH (Fractional Dedicated Physical CHannel) has been incorporated. Currently, the 3GPP standard defines the usage of the F-DPCH as illustrated in FIG. 2A. FIG. 2A shows the F-DPCH slot format as opposed to the regular DPCCH (Dedicated Physical Control CHannel) slot format illustrated in FIG. 2B. As shown in FIG. 2B, the DPPCH slot format, during DTX (Discontinuous Transmission) periods indicated in FIG. 2A, includes TFCI (Transport Format Combination Indicator) and dedicated Pilot bits. By contrast, the F-DPCH carries control information; namely, transport power control bits (TPC) from, for example, the Node B to the UE. Namely, the F-DPCH may be used when all higher layer data, control and user plane, are mapped on the HS-DSCH transport channel, which was specified as part of the release 5 versions of the 3GPP specifications. The only information currently kept on F-DPCH are the TPC bits, which are dedicated to each user and are transmitted once per slot.
As shown in FIG. 2A, a slot of the F-DPCH has a length of 2560 chips that may be divided into ten fields of 256 chips each. By time-aligning the different F-DPCHs of different users (with step sizes of 256 chips), the DTX periods can be used to multiplex the TPC information of different UEs as depicted in FIG. 3. Accordingly, operation of the F-DPCH allows to time multiplex up to 10 users or UEs on a single channelization code; and therefore, makes more efficient use of the code resources compared to the operation on a DPCH (Dedicated Physical CHannel) in earlier releases, where one channelization code had to be utilized per user. Stated another way, a single channelization code may have ten fields, each supporting the F-DPCH of a different user. Thus, ten UEs can be served utilizing a single channelization code for the ten F-DPCHs instead of ten different channelization codes for all UE's F-DPCH.
However, channelization codes are not used for F-DPCHs in this manner for UEs or mobile stations in a soft handoff or handover operation. As is known, during soft handoff, the two or more Node Bs involved in the soft handoff attempt to maintain a timing alignment with respect to the UE. The process for effecting this synchronization is well known and described in, for example, 3GPP TR 25.878. However, even with the timing adjustments made to effect synchronization, drift does occur and the requirement that the TPC bits received from the two different Node Bs fall into a certain reception window (e.g., +/−148 chips) of the UE may not be met. Accordingly, one proposal suggests using an extra channelization code for each extra Node B to UE connection that results during soft handoff. Namely, if the soft handoff involves the original serving Node B and two candidate Node Bs, the RNC will assign two new channelization codes, one for each of the candidate Node Bs to use for F-DPCH transmissions to the UE.